1. Field of the Invention
The present invention relates to a novel electroluminescence (EL) device and more particularly to an organic EL device capable of emitting light in a region of bluish purple to green at a high luminance and in a stabilized manner. Moreover, the present invention relates to novel aromatic dimethylidyne compounds useful, for example, as emitting materials for an EL device, processes for efficiently preparing the above compounds, and an EL device using the above compound.
2. Description of the Related Arts
A device utilizing EL performance of an organic compound has been long studied in view of fluorescence of the organic compound. For example, W. Helfrish, Dresmer, Williams et al. succeeded in emission of blue light using anthracene crystal (J. Chem. Phys. 44, 2902 (1966)). Vincett, Barlow, et al. produced a light emitting device by a vapor deposition method, using a condensed polycyclic aromatic compound (Thin Solid Films, 94, 171 (1982)).
However, only a light emitting device low in luminance and luminous efficiency has been obtained.
It is reported that emission of blue light of 100 cd/m.sup.2 was obtained using tetraphenylbutadiene as a light emitting material (Japanese Patent Application Laid-Open No. 194393/1984). In practice, however, the efficiency is markedly low and is unsatisfactory.
It is reported that a green light emitting organic thin film EL device providing the maximum luminance of more than 1,000 cd/m.sup.2 and an efficiency of 1 lm/W was developed by laminating a diamine compound conveying a hole and a luminous aluminum chelate complex as a light emitting material (Appl. Phys. Lett., 51, 913 (1987)).
It is also reported that a distyrylbenzene compound well known as a laser dye exhibits high fluorescent properties in the region of blue to blue green, and a light emitting material using the distyrylbenzene compound in a single layer form emits EL light of about 80 cd/m.sup.2 (European Patent 0319881).
However, a light emitting material providing light other than green light (particularly blue-based light) in a luminance as high as more than 1,000 cd/m.sup.2 and with high efficiency has not been obtained.
In connection with the structure of the aforementioned organic EL device, those obtained by properly providing a hole injection layer or an electron injection layer into a basic structure having a positive electrode/light emitting layer/negative electrode, e.g., a structure of positive electrode/hole injection layer/light emitting layer/negative electrode, or a structure of positive electrode/hole injection layer/light emitting layer/electron injection layer/negative electrode are known. The hole injection layer functions to inject a hole into the light emitting layer from the positive electrode, and the electron injection layer, to inject an electron into the light emitting layer from the negative electrode. It is known that placing the hole injection layer between the light emitting layer and the positive electrode permits injection of more holes at a lower voltage, and that electrons injected from the negative electrode or the injection layer into the light emitting layer are accumulated at the light emitting layer side in an interface between the light emitting layer and the hole injection layer when the hole injection layer does not have electron transporting ability, increasing a luminous efficiency (Applied Physics Letters, Vol. 51, p. 913 (1987)).
As such organic EL devices, for example, (1) a laminate type EL device having a structure of positive electrode/hole injection layer/light emitting layer/negative electrode in which the light emitting layer is made of an aluminum complex of 8-hydroxyquinoline, and the hole injection layer, of a diamine compound (Appl. Phys. Lett., Vol. 51, p. 913 (1987)), (2) a laminate type EL device having a structure of positive electrode/hole injection zone/organic light emitting zone/negative electrode in which an aluminum complex of 8-hydroxyquinoline is used in preparation of the light emitting zone (Japanese Patent Application Laid-Open No. 194393/1984), and (3) an EL device having a structure of positive electrode/hole injection zone/light emitting zone/negative electrode in which the light emitting zone is made of a host material and a fluorescent material (European Patent Publication No. 281381) are known.
In the above EL devices (1) and (2), although light emission of high luminance is attained at a low voltage, it is necessary to control the temperature of a vapor deposition source not to be more than 300.degree. C., i.e., as low as nearly an evaporation temperature in vapor deposition, because an aluminum complex of 8-hydroxyquinoline when used as a light emitting material is readily decomposable at a temperature of more than about 300.degree. C. It is therefore difficult to control conditions for production of a device and, moreover, vapor deposition speed is decreased. Thus the devices (1) and (2) inevitably suffer from a problem of a reduction in productivity of devices. Moreover the aluminum complex of 8-hydroxyquinoline can emit green light, but not blue light.
In the EL device (3), a compound capable of injecting a hole and a electron from the outside, preferably an aluminum complex of 8-hydroxyquinoline is used as a host material, and as a fluorescent material, a compound capable of emitting light in response to re-combination of a hole and an electron, such as a known fluorescent dye.
In this device, among an injection function (function to inject a hole from either a positive electrode or a hole injection layer and also to inject an electron either from an electrode or a negative electron injection layer, upon application of electric field), a transport function (function to transport a hole and an electron upon application of electric field), and a light emitting function (function to provide a field for recombination of a positive hole and an electron, thereby producing light emission), the light emitting zone (light emitting layer) should have the injection function, the transport function, and part of the light emitting function fulfilled by the host material, while only part of the light emitting function is fulfilled by the fluorescent material. For this reason, the host material is doped with a very small amount (not more than 5 mol %) of the fluorescent material. An EL device of the above structure can emit light in the region of from green to red at a luminance as high as above 1,000 cd/m.sup.2 by application of a voltage of about 10 V.
In this EL device, however, the same problems as in the above EL devices (1) and (2) are encountered, because it usually uses 8-hydroxyquinoline-Al complex as a host material. Moreover, it is impossible to emit light of a short wavelength having a higher energy than the energy gap value of the 8-hydroxyquinone from a fluorescent material; emission of blue light cannot be obtained.
As described above, the above devices (1), (2) and (3) cannot provide blue light emission of high luminance in a stabilized manner and with high efficiency. However, they provides an epoch making technical advance by showing that a high luminous and high efficiency EL device can be realized by selecting a light emitting material with a structure of positive electrode/hole injection layer made of amino derivative/light emitting layer/negative electrode. In this selection of the light emitting material, the three functions of the above light emitting layer should be satisfied. Moreover it should be taken into consideration that a material with excellent film forming properties as a light emitting layer should be selected. Moreover the material selected should have excellent heat resistance properties and should avoid decomposition at the time of heating for vacuum deposition. It has been difficult to find a light emitting material to satisfy all the above requirements. Thus the present inventors made extensive investigations to develop a compound providing light emission in a region of bluish purple to green, particularly in a blue region at a high luminance and with high efficiency.
The present inventors made extensive investigations to attain the above objects. As a result, they have found that stilbene-based compounds having specified structures have an injection ability, a transporting ability and a light emitting ability essential for a light emitting layer, are excellent in heat resistance and thin film forming properties, are free from decomposition even if heated to a vacuum deposition temperature, can form a uniform and dense film having excellent thin film forming properties, and moreover are rarely subject to formation of pinholes at the time of formation of the opposite electrode (metal), and that if the above compounds are used as light emitting materials, an EL device can be obtained with high efficiency and moreover the EL device provides stable light emission of high luminance from bluish purple to green upon application of a low voltage. Based on the findings, these present invention has been accomplished. Furthermore, the EL device is of high efficiency in a practical luminous region (80 to 200 cd/m.sup.2).